Interest in dark chromium deposits has started already with developing chromium deposits from hexavalent chromium due to its high wear and corrosion resistance and high thermal and electrical conductivity. Dark chromium coatings have been used for decorative purposes and as solar radiation absorbing coating for solar collector panels.
Then chromium deposits originating from trivalent chromium came into focus because of its better environmental tolerance. Interestingly, the first commercially applicable trivalent chromium electroplating baths turned out to produce chromium coatings which were already of slightly darker colour than the coatings resulting from hexavalent chromium electroplating baths.
But the color of coatings obtained from trivalent chromium was not dark enough to meet the expectations for decorative parts or satisfy the requirements for solar collectors. A few strategies were developed to produce dark chromium coatings from trivalent chromium which are mainly in the field of solar collectors.
U.S. Pat. No. 4,196,063 to Barnes and Ward relates to trivalent chromium plating baths containing cobalt ions or iron II ions and phosphate ions, alternatively iron III and hypophosphite, which produce black chromium deposits with better electrical and thermal conductivity, better wear resistance and better toughness than black deposits from hexavalent chromium baths.
Selvam et al. (Metal Finishing, 1982, 107-112) performed a systematic investigation on compositions of trivalent chromium baths and conditions of electroplating black chromium coatings from these baths for application in solar thermal devices. Black deposits with properties similar to black deposits resulting from hexavalent chromium plating baths were obtained for bath compositions containing chromium chloride, ammonium chloride and oxalic acid. In addition the authors mention disadvantages of the composition and plating method like formation of chlorine, high consumption of oxalic acid, critical pH control, and nonadherent black deposits.
Abbott et al. (Trans Inst Met Fin, 2004, 82 (1-2), 14-17) report on the possibility to produce a black chromium coating by electrodepositing it from an ionic liquid made of trivalent chromium chloride and choline chloride additionally containing lithium chloride. The black chromium deposits are especially thick, adherent and crackfree and are assumed to have a nanocrystalline structure.
Abdel Hamid (Surface & Coatings Technology 203, 2009, 3442-3449) presents a black chromium deposit on steel which was plated from a solution containing trivalent chromium ions, cobalt ions and hexafluorosilicic acid (H2SiF6) as an oxidizing agent. The resulting layers mainly consisted of chromium, chromium oxide and cobalt oxide. They revealed good absorbance properties for solar energy and good thermal stability and were therefore regarded as suitable for solar thermal applications.
The dark chromium deposits of the above mentioned state of the art present good properties for solar thermal applications. But these dark chromium deposits are not suited for decorative purposes because they are dull, even when deposited on bright surfaces. Actually, for decorative chromium deposits there is a demand for glossy dark chromium coatings.
Further several trivalent chromium electroplating baths containing sulphur compounds are reported.
Patent GB 1431639 to Barclay and Morgan relates to a chromium electroplating solution in which the source of chromium comprises a trivalent chromium-thiocyanate complex. The chromium-thiocyanate complex leads to formation of a bright, relatively hard, uncracked chromium layer with good corrosion resistance and the plating process had a better throwing power and current efficiency than in conventional chromic acid baths.
U.S. Pat. No. 4,473,448 to Deeman refers to electrodeposition of chromium from electrolytes containing trivalent chromium ions and low concentrations of thiocyanate or a spectrum of other sulphur containing compounds. Electroplating a workpiece with these electrolytes gave light colored chromium electrodeposits.
U.S. Pat. No. 4,448,648 to Barclay et al. discloses an electroplating solution for plating chromium from trivalent state. The electroplating solution additionally contains sulphur containing species having a S—S or S—O bond which promote chromium deposition. As a result a lower chromium concentration is needed within the electrolyte.
US Patent application 2010/0243463 relates to an electrolyte and method for decorative chromium coating. The electrolyte also contains sulphur-containing organic compounds. Employing this electrolyte yields chromium-sulfur alloy deposits that are more corrosion resistant especially in environments containing calcium chloride.
US Patent applications US 2009/0114544 A1 and US 2007/0227895 A1 by Rousseau and Bishop disclose a process and an electrodeposition bath for depositing nanogranular crystalline functional chromium deposits. The electrodeposition bath includes trivalent chromium, a source of divalent sulphur, and optionally ferrous ions. Attempts of the present inventors to produce decorative chromium deposits from the described electrolyte T7 containing thiosalicylic acid and ferrous sulphate were not successful.
Actually no deposits could be generated when employing pH values of 2.8 and 4.2 within the electrolyte at current densities of 10, 20, 30 and 40 A/dm2.